Technical Field
The present invention relates to input front-end circuits used when external signals are sent to a switching power supply control IC.
Background Art
Input pre-circuits convert voltage that an integrated circuit (IC) receives an external source such that the voltage is in a range that is appropriate for processing by a circuit within the IC itself. FIG. 5 shows the first of two example configurations of a conventional input front-end circuit for a switching power supply control IC. In addition, FIG. 6 shows an example configuration of a general switching power supply device configured to include a switching power supply control IC 8.
In FIGS. 5 and 6, respectively, the reference character 1 represents an AC input (input voltage VAC), 2 represents a transformer for forming an input filter, 3 represents a capacitor for forming the input filter, 4 represents a diode bridge for rectifying the AC input, 5 represents a capacitor for removing switching-induced ripple and noise, 6 represents a diode for changing the AC input into an input voltage for the control IC using half-wave rectification, 7 represents a resistor for limiting input current to the control IC, 8 represents the control IC, 9 represents a thermistor connected to a LAT terminal of the control IC for latch shutdown-type overheat protection, 10 and 11 represent a capacitor and a resistor, respectively, for forming a noise filter of a CS terminal, 12 represents a sense resistor for converting the ON current of a MOSFET into a voltage, 13 represents a capacitor for holding the voltage of a VCC terminal, which is the power supply of the control IC, 14 represents a diode for preventing a reverse current from flowing from the VCC terminal to an auxiliary winding, 15 represents the auxiliary winding of a transformer for supplying the power supply to the control IC when in operation, 16 represents a primary winding of the transformer, 17 represents the MOSFET for switching, 18 represents a secondary winding of the transformer, 19 represents a diode for rectifying the secondary-side voltage, 20 represents a capacitor for smoothing the secondary-side voltage, 21 represents a photocoupler for sending secondary-side load information to the primary-side, 22 represents a shunt regulator for converting the output voltage state into a current, 23 and 24 represent resistors for dividing the output voltage, and 25 represents a secondary-side output and a terminal connected to a load (not shown).
The IC input terminal shown in FIG. 5 corresponds to a VH terminal of the control IC 8 in FIG. 6. The VH terminal monitors the input voltage and monitors the input voltage peak or whether or not the input voltage is being applied in order to make the function block within the control IC correct circuit characteristics in accordance with the input voltage.
Furthermore, the VH terminal makes the function block within the control IC carry out functions such as suspending the operations of the control IC when the peak voltage decreases or suspending operations until at least a prescribed low voltage is reached when the input voltage is being applied.
When the input voltage to be monitored is from a worldwide power supply, due to the input specifications being 85 Vac to 264 Vac, the VH terminal input voltage rectified by the rectifier diode 6 shown in FIG. 6 can be 0 Vdc to approximately 120 Vdc or even 0 Vdc to 380 Vdc.
In order to receive and process the input voltage from a worldwide power supply using a single control IC, it is necessary to assume that a voltage of 0 Vdc to 400 Vdc will be received. On the other hand, for control ICs that receive signals, a voltage-dividing resistor is prepared within the input terminal, and voltages of 0 Vdc to 400 Vdc are converted into voltages capable of being processed within the control IC. The internal power supply of the control IC has a large division ratio such as 1:250 because the internal power supply is at 5V (recently 3.3V), which is much lower than the voltages within the input voltage range.
When the input voltage is high, the divided voltage is 1V to less than 2V and is sufficient for a range of internal circuit operations, but when a low input voltage is detected, for example when 45 Vdc is detected, the signal is extremely low at 0.18V and close to the lower limit of the input range for the receiving-side internal circuit of the control IC, thereby mixing with noise within the control IC and making it difficult to distinguish between the signal and the noise, which causes the control IC to malfunction and/or falsely detect.
For the same reason, it also becomes difficult to define, with high accuracy, the circuit characteristics using the control IC specifications.
Note that, in order to solve these problems, a plurality of division ratios can be prepared in accordance with the voltage to be detected, or a voltage-divided signal is amplified using an AMP 103 or the like for each function in accordance with the voltage to be detected as shown in the second of two example configurations of an input front-end circuit 100 in a switching power supply control IC of FIG. 7.
However, for the second of two example configurations of the input front-end circuit 100 shown in FIG. 7, when a function for function block A to monitor the entire input range to control constant-switching pulse and a function for function block B to switch to a control mode using an approximate specific low input voltage are provided, a configuration is adopted for amplifying the input voltage using the AMP 103 or the like, because information on high input voltage is not required for function block B. For example, such a configuration was applied to the MUL terminal shown in FIGS. 2 and 4 of Patent Document 1 and to the VH terminal shown in FIG. 1 of Patent Document 2.
However, using such a configuration may be inconvenient due to problems such as errors from the AMP 103 being applied to the input part of function block B.
In other words, it is inconvenient that a function group for executing a plurality of functions based on a single input does not act together based on information on a plurality of inputs. This inconvenience is allowed as long as the control mode switching points are only slightly unaligned, but in a worst-case scenario, the control mode switching needs an excessive amount of responses from the switching control, potentially causing hunting.
The present invention addresses a first problem of removing the above inconvenience.
Next, the present invention addresses a second problem of not receiving voltage having a significant amplitude such as an AC voltage as the input voltage. These problems are described below.
Power-factor correction control ICs and the like generally control switching pulses in accordance with the input AC voltage, but there are recent control schemes that monitor the entire circuit current of the switching power supply instead of the input voltage.
For these schemes, the sinusoidal behavior of the input voltage appears in the peak current of the circuit current, is converted into a voltage by a sense resistor, and is then received by the control IC.
In this configuration, the current-sensing resistor is extremely small in order to minimize energy loss, and as a result, the voltage that the control IC receives is low. For power-factor correction control ICs in particular, it is necessary to detect the timing at which the circuit current crosses zero using the IS terminal shown in FIG. 1 of Patent Document 3, for example; thus, it is difficult to handle miniscule voltages, which may lead to malfunctions.